1. Field of the Invention
The present invention relates to an antenna device and a portable radio set, and more particularly, is suitably applied to a cellular telephone.
2. Description of the Related Art
The cellular telephone of this type has been decreased in size and weight so far in order to improve the portability. Thereby, a retract/pull-out type of whip antenna device is positively developed as an antenna device provided for a cellular telephone. There is a cellular telephone configured as shown in FIGS. 1A and 1B as the cellular telephone of the above type.
In case of a cellular telephone 1 having the above configuration, a whip antenna device 3 is providing for in a housing 2 made of a nonconductive material such as synthetic resin.
The antenna device 3 has an antenna section 6 provided with a rod antenna 4 made of a conductive wire rod and a helical antenna 5 formed by helically winding a conductive wire rod. The antenna section is set so as to be freely retracted and pulled out along a direction in which the antenna section 6 is pushed into the housing 2 shown by an arrow a at the upper end 2A of the housing 2 (this direction is hereafter referred to as retracting direction) and inversely, along a direction in which the section 6 is pulled out of the housing 2 (this direction is hereafter referred to as pull-out direction).
In the antenna section 6, a first power-supply member 7 made of a conductive material and having a protrusion 7A is electrically and mechanically connected to the lower end of the rod antenna 4 and a joint 8 made of a nonconductive material is mechanically connected to the upper end of the rod antenna 4.
Moreover, a second power-supply member 9 made of a conductive material is electrically and mechanically connected to the lower end of the helical antenna 5 and mechanically connected to the joint 8. Thereby, in the antenna section 6, the rod antenna 4 and the helical antenna 5 are mechanically connected each other by the joint 8 but they are electrically separated from each other.
Moreover, the rod antenna 4 is covered with a rod antenna cover 10 and the helical antenna 5 is stored in a cap-shaped helical antenna cover 11 so that the antennas 4 and 5 do not directly contact a user.
A circuit board (not illustrated) on which various circuit devices including a transceiving circuit 12 and a matching circuit 13 are mounted and a shielding case serving as a ground member made of a conductive material for covering the circuit board are stored in the housing 2.
Moreover, an antenna power-supply terminal 14 made of a conductive material electrically connected to the matching circuit 13 is set inside of the upper end 2A of the housing 2 and only either of the rod antenna 4 and helical antenna 5 is electrically connected to the antenna power-supply terminal 14 when the antenna section 6 is retracted or pulled out.
Actually, in the antenna device 3, the antenna section 6 is pushed in and the helical antenna cover 11 is pushed in the retracting direction and made to contact the upper end 2A of the housing 2 to push the rod antenna 4 into the housing 2 and store the rod antenna 4 in the housing 2 and electrically connect the second power-supply member 9 to the antenna power-supply terminal 14.
Moreover in the antenna device 3, power is supplied to the helical antenna 5 from the transceiving circuit 12 through the matching circuit 13, antenna power-supply terminal 14, and second power-supply member 9 in order under the above state to make the helical antenna 5 operate as an antenna.
Furthermore, in the antenna device 3, by electrically separating the rod antenna 4 from the antenna power-supply terminal 14 by the joint 8, the rod antenna 4 is not operated as an antenna.
In the antenna device 3, however, when the second antenna cover 11 is pulled in the pull-out direction while the rod antenna 4 is stored in the housing 2, the rod antenna 4 is pulled out to the outside from the upper end 2A of the housing 2, the protrusion 7A of the first power-supply member 7 is made to contact the antenna power-supply terminal 14, and thereby the first power-supply member 7 is electrically connected to the antenna power-supply terminal 14.
Furthermore, in the antenna device 3, power is supplied to the rod antenna 4 from the transceiving circuit 12 through the matching circuit 13, antenna power-supply terminal 14, and first power-supply member 7 in order under the above state to make the rod antenna 4 operate as an antenna.
Furthermore, in the antenna device 3, by electrically separating the helical antenna 5 from the antenna power-supply terminal 14 by the joint 8, the antenna 5 is not operated as an antenna.
In this connection, to make the rod antenna 4 and helical antenna 5 operate as antennas, the impedances of the rod antenna 4 and helical antenna 5 are matched with the impedance of the unbalanced transmission line 16 by the matching circuit 13.
Moreover, the shielding case functions as ground for various circuit devices and moreover functions as an electrical shielding plate for preventing radio waves of external noise and radio waves emitted from the antenna section 6 from entering various circuit devices mounted on a circuit board.
Thereby, the cellular telephone 1 makes it possible to, at the time of pulling out the antenna section 6, transmit a transmission signal configured of a high-frequency signal from the transceiving circuit 12 to the rod antenna 4 through the matching circuit 13, transmit the transmission signal to a base station (not illustrated) through the rod antenna 4, and transmit a reception signal configured of a high-frequency signal transmitted from the base station and received by the rod antenna 4 to the transceiving circuit 12 through the matching circuit 13.
Moreover, the cellular telephone 1 makes it possible to prevent damage to the rod antenna 4 by storing the antenna 4 in the housing 2 at the time of retracting the antenna section 6, transmitting a transmission signal from the transceiving circuit 12 to the helical antenna 5 through the matching circuit 13 under the above state, transmitting the transmission signal to a base station through the helical antenna 5, and transmitting a reception signal transmitted from the base station and received by the helical antenna 5 to the transceiving circuit 12 through the matching circuit 13.
The cellular telephone 1 having the above configuration is provided with an unbalanced transmission line 26 configured of a microstrip line formed on a circuit board, in which the rod antenna 4 or helical antenna 5 is electrically connected to the transceiving circuit 12 through the hot side of the unbalanced transmission line 26 and the matching circuit 13 sequentially, and the ground side of the unbalanced transmission line 26 is grounded to the shielding case.
Then, in the cellular telephone 1, as shown in FIGS. 2A and 2B, when power is supplied to the rod antenna 4 and helical antenna 5 through the hot side of the unbalanced transmission line 26 to operate the rod antenna 4 and helical antenna 5 as antennas, a leakage current i1 flows to the shielding case 27 having a potential almost equal to that of the ground side of the unbalanced transmission line 26 from the ground side and thereby, the shielding case 27 also operates as an antenna.
Therefore, in the cellular telephone 1, when the shielding case 27 operates as an antenna and the housing 2 is held by a hand of a user or when the housing 2 is approached to the head of the user, antenna characteristics of the cellular telephone 1 are deteriorated because the shielding case 27 approaches a hand or the head of the user through the housing 2.
When the antenna section 6 is pulled out, the physical length of the rod antenna 4 pulled out from the upper face 2A of the housing 2 is comparatively large and it is possible to separate the rod antenna 4 from a hand of a user holding the housing 2 or the head of the user to which the housing 2 is approached. Therefore, deterioration of antenna characteristics of the cellular telephone 1 is almost caused by the shielding case 27 approached to a user.
However, when the antenna section 6 is retracted, the physical length of the helical antenna 5 protruded beyond the upper face 2A of the housing 2 is very small as compared with the case of the pulled-out rod antenna 4 and the helical antenna 5 greatly approaches a hand of the user holding the housing 2 or the head of the user to which the housing 2 is approached together with the shielding case 27. Therefore, antenna characteristics of the cellular telephone 1 are extremely deteriorated compared to the case in which the antenna s;section 6 is pulled out and as a result, a problem occurs that communication quality is deteriorated.
Moreover, the cellular telephone 1 has a problem that when the antenna section 6 is retracted, the shielding case 27 and helical antenna 5 are made to approach a hand or the head of a user and thereby, the power for unit time or unit mass absorbed in a specific portion of a user {so-called Specific Absorption Rate (SAR)) is increased.
In view of the foregoing, an object of the invention is to provide an antenna device and a portable radio set capable of greatly reducing the deterioration of communication quality when an antenna element is retracted.
The foregoing object and other objects of the invention have been achieved by the provision of an antenna device comprising a first antenna element provided so as to be freely retracted or pulled out, a second antenna element, an unbalanced transmission line for supplying power to the first and second antenna elements, balanced-to-unbalanced transform means for performing balanced-to-unbalanced transform between the unbalanced transmission line on one hand and the first and second antenna elements on the other, and connection means for electrically connecting the first and second antenna elements to the balanced-to-unbalanced transform circuit when the first antenna element is retracted and electrically connecting at least the first antenna element to the balanced-to-unbalanced transform circuit when the first antenna element is pulled out, in which power is supplied to the first and second antenna elements from the unbalanced transmission line through the balanced-to-unbalanced transform means so as to operate the first and second antenna elements as antennas when the first antenna element is retracted and power is supplied to at least the first antenna element from the unbalanced transmission line through the balanced-to-unbalanced transform means to operate the first antenna element as an antenna when the first antenna element is pulled out.
As a result, when the first antenna element is retracted, it is possible to prevent a leakage current from flowing through a ground member to which the unbalanced transmission line is grounded from the first or second antenna element through the unbalanced transmission line due to balanced-to-unbalanced transform by the balanced-to-unbalanced transform means, thereby preventing the ground member from operating as an antenna, and greatly reducing the deterioration of antenna characteristics near a human body.
Moreover, the present invention provides a portable radio set having an antenna device comprising a first antenna element provided so as to be freely retracted and pulled out, a second antenna element, an unbalanced transmission line for supplying power to the first and second antenna elements, balanced-to-unbalanced transform means for performing balanced-to-unbalanced transform between the unbalanced transmission line on one hand and the first and second antenna elements on the other, and connection means for electrically connecting the first and second antenna elements to the balanced-to-unbalanced transform circuit when the first antenna element is retracted and electrically connecting at least first antenna element to the balanced-to-unbalanced transform circuit when the first antenna element is pulled out, in which power is supplied to the first and second antenna elements from the unbalanced transmission line through the balanced-to-unbalanced transform means when the first antenna element is retracted to operate the first and second antenna elements as antennas and power is supplied to at least the first antenna element from the unbalanced transmission line through the balanced-to-unbalanced transform means when the first antenna element is pulled out to operate the first antenna element as an antenna.
As a result, when the first antenna element is retracted, it is possible to prevent a leakage current from flowing through a ground member to which the unbalanced transmission line is grounded from the first or second antenna element through the unbalanced transmission line due to balanced-to-unbalanced transform by the balanced-to-unbalanced transform means, and thereby, prevent the grounding member from operating as an antenna and greatly reduce the deterioration of antenna characteristics nearby a human body.
The nature, principle and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings in which like parts are designated by like reference numerals or characters.